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2283 (8776) Dark Dagger Acronicta tridens ([Denis & Schiffermüller], 1775) Common 2284 (8777) Grey Dagger Acronicta psi (Linnaeus, 1758) Common

Diagnostic external characters Although quite distinct from all other resident British species in colour and markings, psi and tridens are virtually identical and show a similar range of variation. It is therefore, not considered safe to record them without examination of genitalia. However, certain trends exist (although there is considerable overlap) which can sometimes be used in the field to select likely specimens of each species for closer examination, if recording presence only, as described A by Waring et al . (2009).

Diagnostic morphological characters of the males A With practice, the number of arms on the ventral surface of the valva can be seen on live moths by very gently pinching the tip of the abdomen and inspecting with a hand-lens. On freshly killed or anaesthetised moths this can be achieved by squeezing gently on the abdomen with a setting needle to extrude the valvae.

Ventral surface of valva with three protruding arms (Fig. 92, A) ...... tridens

Ventral surface of valva with two protruding arms (Fig. 94, A) ...... psi

Diagnostic morphological characters of the females Appendix bursae larger (Fig. 93, A) ...... tridens 92. Acronicta tridens male 93. Acronicta tridens female

Appendix bursae smaller (Fig. 95, A) ...... psi

Larvae When well-grown the larvae are quite distinct and provide a further means of distinguishing these two species. The chief difference is that psi has a broad yellow dorsal stripe (which becomes white before pupation), whereas in tridens the dorsal stripe is orange and white with a fine black line running through the centre. They are described in greater detail and illustrated by Porter (1997) and also illustrated by Waring et al . (2009). Females of both species are less frequently encountered, but if gravid willingly oviposit in captivity and the larvae are not difficult to rear.

Other similar species Although not reliably recorded in the British Isles, Large Dagger Acronicta cuspis (Hübner) occurs on the near A A continent and is very similar in appearance, such that immigrants could be overlooked as atypical psi or tridens .

Compared to tridens and psi , cuspis is stated to be usually larger, with the black basal streak and tornal dagger-mark on forewing also usually larger and broader. In the male, the hindwing is darker than in tridens and psi , being grey-brown (white or whitish in male tridens and ps i). The male genitalia of cuspis are very similar to those of tridens , with 3 arms extending ventrally on the valva. However, in cuspis the distal half of the ventral margin of the sacculus is curved (straight in tridens ) sweeping out in an arc to the ventral arm, which is longer than in tridens . In the female, the appendix bursae is stated to be larger than in tridens and psi . Fibiger et al . (2009) describe and compare external and genitalic features of the three species in detail. Their male specimen of tridens appears to have a rather blunt ending on the middle of the three valva arms, this arm normally being tapered and hooked at the apex. 94. Acronicta psi male 95. Acronicta psi female

If a female is suspected of being cuspis , eggs should also be obtained and adults reared. The larva of cuspis resembles that of psi , but compared to that species the anterior hump is shorter and the hairs arising from it are longer. Therefore, a combination of yellow-striped larvae and three arms of the valva of the males is diagnostic for cuspis .

Plate 26. Genitalia of Acronicta tridens and Acronicta psi (aedeagus not shown).